1. Field of the Invention
The present invention relates to a converter lens that is attached in front of (on the object side of) an imaging lens (a main lens system) of a camera such as a digital still camera, a camcorder, or a silver salt (film) camera so as to change the focal length of the entire lens system.
2. Description of the Related Art
Japanese Patent Laid-Open No. 55-32046 discloses a front teleconverter lens (teleconverter) that is attached in front of an imaging lens so as to increase the focal length of the entire system.
Most teleconverters include a front unit having positive optical power and a rear unit having negative optical power. The distance between the principal points of the two lens units is the sum of focal lengths of the two lens units. The two lens units together constitute an afocal system. Optical power is the reciprocal of the focal length of a lens unit. In the case of a lens unit composed of a refractive optical element such as a lens, optical power is equal to refractive power. The simplest lens system may be composed of a single positive lens and a single negative lens. However, in the case of only two lenses, aberration correction is difficult, and high optical performance cannot be obtained.
Japanese Patent Laid-Open No. 4-191717 and Japanese Patent Laid-Open No. 6-289289 disclose teleconverters including three or more lenses.
In the case of a high afocal magnification, when a front unit is composed of a single lens, correction of aberrations is difficult. In order to solve this problem, the teleconverter disclosed in Japanese Patent Laid-Open No. 4-191717 includes a front unit composed of two positive meniscus lenses, each lens having weak optical power. However, the rear unit is composed of a single negative lens, and therefore correction of chromatic aberration is insufficient.
The teleconverter disclosed in Japanese Patent Laid-Open No. 6-289289 includes a front unit composed of a single positive lens formed of low-dispersion glass so as to prevent deterioration of chromatic aberration. However, the afocal magnification is low (less than 1.3 times).
The lenses constituting a front unit have a large diameter. Therefore, if a front unit is composed of too many lenses, the weight and the cost increase significantly.